ESTRO 2021 Abstract Book

S1646

ESTRO 2021

Purpose or Objective Despite good survival rates in breast cancer, the development of therapy resistances during treatment remains a major problem. This is especially true for recurrent tumors, which often display more resistances than the primary tumor. Targeted inhibition of resistance-associated factors and thereby resensitizing the tumor cells to radio- and chemotherapy seems to be a promising therapeutic approach. Therefore this project aims to identify factors involved in DNA damage repair and replication processes which are known to play key roles in the cellular resistance mechanisms. Materials and Methods Patient-derived organoids and isogenic MCF7 clones with different BRCA1 mutations induced by CRISPR/Cas9 were grown in 3D cell culture. For validation of the 3D architecture, the organoids were stained with hematoxylin and eosin. As the BRCA1 level has a direct impact on homologous recombination (HR) and radioresistance, the survival of the clones and also the patient-derived organoids after irradiation up to 12 Gy was monitored using colony formation assay. Additionally, the DNA fiber assay was employed to investigate the replication stress level after irradiation with 6 Gy. Results The histological analysis revealed a successful growth of the patient-derived organoids (PDOs) and 3D organized spheroids of the isogenic MCF7 clones. As expected, the radiosensitivity was dependent on the BRCA1 expression level in the MCF7 clones. The PDOs showed varying survival, not dependent on the molecular subtype of the parental tumor. Further, the replication stress level represented by fork stalling after 6 Gy irradiation was significantly increased in all cell lines and PDOs (p < 0.0001). Strikingly, the sensitivity to irradiation correlated with the increase in the replication stress level after 6 Gy (R 2 = 0.81). Based on the results of the survival analysis and replication stress level after irradiation the clones and PDOs were classified as radio-resistant or sensitive. Currently we are investigating combinations of clinical relevant drugs to test for sensitizing effects against radiation in the resistant subgroup. The next step then will be to determine the involved factors by conducting different methods like the PAMgene technology or transcriptome analysis. Conclusion Our data show, that we can identify radio-resistant or -sensitive tumors by monitoring the survival and replication stress level after irradiation. This classification enables the targeted analysis of factors (e.g. kinases) in radioresistant cells, thus, providing possible new targets to resensitize the cells to radiotherapy. Clinically, this would offer a new approach for the treatment of radioresistant tumors.

PO-1932 Cell cycle arrest induced by AsiDNA in normal cells: a possible mechanism for radioprotection?

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Digital Poster: Radiation response biomarkers

PO-1933 Can baseline or Ra-223-induced changes in the plasma predict progressive disease mCRPC patients? S. Lunj 1 , Y.P. Song 2 , A. Hudson 2 , K. Patel 2 , H. Nightingale 2 , T. Smith 1 , P. Hoskin 1 , R. Bristow 1 , C. West 1 , A. Choudhury 1,3 1 The University of Manchester, Division of Cancer Sciences, Manchester, United Kingdom; 2 The Christie NHS Foundation Trust, Clinical Oncology, Manchester, United Kingdom; 3 The Christie NHS Foundation Trust, Division of Cancer Sciences, Manchester, United Kingdom Purpose or Objective Bone targeting agents such as Radium-223 (Ra-223) have changed the landscape for managing metastatic castration- resistant prostate cancer (mCRPC). Ra-223 is approved for men with symptomatic bone metastases but no known visceral metastases. Not all patients respond to Ra-223, thus early stratification of patients can aid personalisation of treatment plans. Our study aimed to explore the feasibility of measuring baseline and early Ra-223 induced changes in circulating cytokines, chemokines and growth factors involved in the immune response. Materials and Methods Fifty patients with mCRPC with bone disease were recruited into the CAPRA study and received up to six four weekly injections of Ra-223 ( Figure 1 ). Patients were labelled responders (R) or non-responders (NR) depending on whether they had improvement or progression of symptoms. Plasma was isolated from whole blood immediately after collection, then frozen and stored at -80 o C. Using a 30-plex Luminex assay, a technology that detects fluorescence (MFI) to estimate the concentration of protein, we identified plasma-based cytokines, chemokines and growth factors and compared the levels between Ra-223 responders and non-responders.